This invention relates to windows that are transparent to infrared radiation, and, more particularly, to an antireflection, damage-resistant coating for such windows.
An infrared sensor is a device that is sensitive to radiation in the infrared range, about 8-11 micrometers wavelength. This wavelength is not visible to the human eye, but is associated with the production of heat. The sensor receives this radiation as an input and produces an electrical output that can be processed into information about the scene viewed by the sensor. Such sensors can be used in a wide variety of situations of interest. They may be passive devices that are useful at night and in other conditions that may render sensors in the visible region ineffective, do not themselves radiate energy, and are relatively light in weight.
One of the important applications of infrared sensors is in flight vehicles such as missiles and aircraft. In this application, the infrared sensor is typically fabricated as an array of individual infrared-sensitive detectors, whose outputs are processed and combined together to form an infrared image of the field of view. The sensor is usually mounted in the nose or belly of the vehicle and faces fowardly into its flight path.
The sensor formed as an array of infrared-sensitive detectors is relatively resistant to damage by shock and vibration. However, it may be easily damaged by abrasion and inpact of objects against the detector elements. Abrasion causes scratching of the surface, and predominates during low-speed flight. When the sensor faces fowardly and moves at speeds of hundreds of miles per hour, the impact of almost any object may fracture the sensor. For example, dust particles in the air, particularly near the surface of the earth, and even rain drops can act as projectiles that pit or fracture the surface of the sensor.
To alleviate this problem, the infrared sensor is protected by a window through which it receives the infrared radiation. The window must be highly transparent to the infrared radiation wavelengths of interest, must itself be resistant to abrasion and damage by particles in the air, and must retain its structural integrity. The window must also be capable of withstanding the temperatures to which it is subjected during flight. When the vehicle flies slowly, as in the case of a helicopter, the window remains relatively cool. When the vehicle flies faster, as in the case of a missle or jet aircraft, the window is heated by aerodynamic heating and may reach temperatures of as much as 150 C.-300 C.
Most infrared-transparent materials used to fabricate the windows are semiconductors, as certain members of this class offer the greatest transmission of infrared radiation. Germanium windows are preferred for use at temperatures below about 100 C. However, the transmittance of germanium is reduced at higher temperatures, due to its intrinsic semiconductor nature.
Other window materials have been identified for use at higher temperatures. At the present time, a zinc salt-based structure is preferred for use in high-temperature infrared windows. The zinc salt structure, typically a layered structure of zinc sulfide and zinc selenide, is fabricated into a window (or dome) form and placed over the sensor. This window has good infrared transmission in the 8-11 micrometer infrared wavelength range at temperatures over 100 C., and up to as much as 150 C.-300 C.
The zinc-salt window is, however, sensitive to abrasion damage and impact damage by dust, sand, other particles, and rain droplets. It must therefore be protected by a hardened exterior surface coating. The coating materials are typically less transparent to infrared radiation than the zinc salts and therefore cannot be used for the entire window construction. The coating is applied as a thin layer so that its total attenuation of the infrared energy is acceptably low.
In one prior approach, diamond-like carbon is applied to the outer surface of the zinc-salt window as a protective coating. Diamond-like carbon is hard and damage resistant, and infrared transparent. However, the diamond-like carbon does not adhere well to the surface of the window when applied in a sufficient thickness to be useful for most missions. There is a tendency for the protective coating to delaminate from the surface of the window during flight under impact conditions of dust or droplets, with the result that the window itself may be rendered insufficiently transparent.
There is therefore a need for an improved window that is highly infrared transparent to infrared radiation. Stated alternatively, the window must have a low reflectance of the infrared energy. The window must be resistant to damage when propelled through the air at low and high speeds. The window must also be serviceable at high temperatures of up to as much as 150 C.-300 C. The provides related advantages.